Optical lithography nowadays often uses wavelengths of 248 nm or 193 nm. With 193 nm immersion lithography, integrated circuit (IC) manufacturing is possible down to 45 nm node, or even down to 32 nm node. However for printing in sub-32 nm half pitch node, this wavelength is probably not satisfactory due to theoretical limitations, unless double patterning is used.
Instead of using wavelengths of 193 nm, a more advanced technology has been introduced, which may be referred to as extreme ultraviolet lithography (EUV lithography). EUV lithography typically uses wavelengths of 10 nm to 14 nm, often 13.5 nm. This technique was previously also known as soft X-ray lithography, which typically uses wavelengths in a wider range of 2 nm to 50 nm.
For optical lithography using wavelengths in the deep ultra violet (DUV) range, the electromagnetic radiation is transmitted by most materials, including glass used for conventional lenses and masks. For optical lithography using shorter wavelengths however, including EUV and soft X-ray lithography, the electromagnetic radiation is absorbed by most materials, including glass used for conventional lenses and masks.
For at least this reason, a completely different imaging system is typically used to perform EUV lithography than is used to perform conventional optical lithography. Instead of using lenses, such an imaging system typically relies on all-reflective optics. That is, such an imaging system is typically composed of reflective optical elements, also referred to as catoptric elements, such as mirrors. These reflective optical elements typically are coated with multilayer structures designed to have a high reflectivity (e.g., up to 70%) at the 13.5 nm wavelength. Furthermore, as air will also absorb EUV light, such an imaging system typically includes a vacuum environment.
In addition to the new imaging system, EUV lithography has introduced a number of other challenges as well. One of these challenges relates to the flatness requirement of the chucked mask (or reticle). Mask flatness requirements are stringent, as even small amounts of non-flatness may cause unacceptable overlay errors to occur. According to SEMI EUV mask and mask chucking standards (P37, the specification for EUV lithography masks substrates and P40, the specification for mounting requirements and alignment reference location for EUV lithography masks), the mask flatness requirements are set at a mask flatness error less than 30 nm peak-to-peak valley for the 22 nm technology node and beyond.
One option for meeting these stringent requirements for mask flatness is an electrostatic chuck. The electrostatic chuck has been optimized for mask blank flatness control. Offline interferometric metrology tools have been developed to enable electrostatic chucking experimentation, as disclosed by, for example, Shu (see Proceedings of SPIE Vol. 6607, 2007). Further, Nataraju et al. disclose performing flatness measurements of the chucked mask using a Zygo interferometer, and have introduced in a finite element model to determine the geometry of the reticle and chuck surface (see Proceedings of SPIE Vol. 6517, 2007). However, these models do not incorporate the chuck properties which may alter from tool to tool. Further, both examples are ex situ, meaning they take place outside the lithographic tool measurement techniques.
An interferometer may be used to determine the flatness of an optical element. The technique is based on emitting light from an interferometric light source and measuring the interference pattern of the reflected light from the mask and from a reference object by a detector. However, in this technique both a test or reference object and a detector are necessary in a lithographic environment. Further, this technique has never been applied to qualify the overall flatness of the mask in situ in a lithographic system.
An example system with improved mask flatness in a lithographic tool is disclosed by Vernon (see United States Patent Application Pub. No. 2008/0079927). According to Vernon, a holder may be used to carry a lithographic mask in a flattened condition. The holder comprises a plurality of independently controllable actuators coupled to the substrate and to the lithographic mask to flatten the lithographic mask.
Further, an example system for evaluating mask flatness in EUV lithography is disclosed by LaFontaine et al. (see U.S. Pat. No. 6,950,176). The system comprises a contactless capacitance probe. By scanning the chucked EUV mask with the capacitance probe, a flatness profile may be determined. However, in this method the mask has to be completely scanned in order to get an overview of the flatness of the mask. Further, an ultra-high vacuum (UHV) capacitance gauge needs to be installed in situ without interfering the present EUV exposure tools during EUV exposure.